© 2008 Cisco Systems, Inc. All rights reserved.Cisco ConfidentialPresentation_ID 1 Routing Talbles Routing Protocols 7.0

Presentation_ID 2 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential

Presentation_ID 3 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Parts of an IPv4 Route Entry Routing Table Entries The routing table hierarchy in Cisco IOS was originally implemented with the classful routing scheme. Although the routing table incorporates both classful and classless addressing, the overall structure is still built around this classful scheme.

Presentation_ID 4 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Parts of an IPv4 Route Entry Directly Connected Entries Two routing table entries are automatically created when an active router interface is configured with an IP address and subnet mask.

Presentation_ID 5 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Routing table entries for the directly connected network Directly connected interfaces have two route source codes. C identifies a directly connected network. L identifies that this is a local route. Local routes and Directly connected networks are automatically created whenever an interface is configured with an IP address and activated.

Presentation_ID 6 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Parts of an IPv4 Route Entry Remote Network Entries Identifies from when the route was last heard.

Presentation_ID 7 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Common Codes for Remote Networks S - Identifies that the route was manually created by an administrator to reach a specific network. This is known as a static route. D - Identifies that the route was learned dynamically from another router using the EIGRP routing protocol. O - Identifies that the route was learned dynamically from another router using the OSPF routing protocol. R - Identifies that the route was learned dynamically from another router using the RIP routing protocol.

Presentation_ID 8 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential

Presentation_ID 9 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Dynamically Learned IPv4 Routes Routing Table Terms Routes are discussed in terms of:  Ultimate route  Level 1 route  Level 1 parent route  Level 2 child routes The Cisco IP routing table is not a flat database. The routing table is actually a hierarchical structure that is used to speed up the lookup process when locating routes and forwarding packets. Within this structure, the hierarchy includes several levels.

Presentation_ID 10 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Dynamically Learned IPv4 Routes Ultimate Route It’s “ultimately” going somewhere An ultimate route is a routing table entry that contains either a next-hop IPv4 address or an exit interface. Directly connected, dynamically learned, and local routes are ultimate routes.

Presentation_ID 11 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Dynamically Learned IPv4 Routes Level 1 Route Do buttons on

Presentation_ID 12 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential A level 1 route is a route with a subnet mask equal to or less than the classful mask of the network address. Therefore, a level 1 route can be a: Network route - A network route that has a subnet mask equal to that of the classful mask. Supernet route - A supernet route is a network address with a mask less than the classful mask, for example, a summary address. Default route - A default route is a static route with the address /0. The source of the level 1 route can be a directly connected network, static route, or a dynamic routing protocol. Dynamically Learned IPv4 Routes Level 1 Route

Presentation_ID 13 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential

Presentation_ID 14 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential A level 1 parent route is a level 1 network route that is subnetted. A parent route can never be an ultimate route. Dynamically Learned IPv4 Routes Level 1 Parent Route

Presentation_ID 15 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Dynamically Learned IPv4 Routes Level 1 Parent Route Parent route is a heading It is NOT an ultimate route (no destination) In the routing table, it basically provides a heading for the specific subnets it contains. Each entry displays the classful network address, the number of subnets and the number of different subnet masks that the classful address has been subdivided into.

Presentation_ID 16 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential A level 2 child route is a route that is a subnet of a classful network address. Dynamically Learned IPv4 Routes Level 2 Child Route

Presentation_ID 17 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Dynamically Learned IPv4 Routes Level 2 Child Route Child of Parent route Is an ultimate route Like a level 1 route, the source of a level 2 route can be a directly connected network, a static route, or a dynamically learned route. Level 2 child routes are also ultimate routes.

Presentation_ID 18 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential

Presentation_ID 19 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Router Lookup Process the router examines level 1 network routes for the best match with the destination address of the IPv4 packet. 1. If the best match is a level 1 ultimate route, then this route is used to forward the packet.

Presentation_ID 20 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Router Lookup Process 2. If the best match is a level 1 parent route, proceed to the next step. the router examines child routes (the subnet routes) of the parent route for a best match. 3. If there is a match with a level 2 child route, that subnet is used to forward the packet. 4. If there is not a match with any of the level 2 child routes, proceed to the next step.

Presentation_ID 21 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential

Presentation_ID 22 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential Router Lookup Process 5. If there is now a lesser match with a level 1 supernet or default routes, the router uses that route to forward the packet. 6. If there is not a match with any route in the routing table, the router drops the packet.

Presentation_ID 23 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential The IPv4 Route Lookup Process Best Route = Longest Match

Presentation_ID 24 © 2008 Cisco Systems, Inc. All rights reserved.Cisco Confidential The IPv4 Route Lookup Process Activity - Determine the Longest Match Route